Flame-retardant injection-molding resins consisting of a low-molecular-weight styrene resin and a low-polymerization-degree vinyl chloride resin are superior materials in view of the mechanical properties and the economical efficiency thereof, and recently are being used as a housing material of OA apparatuses and the like. These flame-retardant resins, however, have disadvantages that burned spots and flashes are liable to be formed in articles molded therefrom and thus the procedure for molding the resins is not simple. This is considered to be caused by the fact that the vinyl chloride resin raises the melt-viscosity of the flame-retardant resin during injection molding, resulting in a larger heat generation in the molding process by shearing forces created by the metal of the screw, the cylinder, and the mold of an injection machine, thus giving rise to thermal decomposition of the resin to form the burned spots and flashes. The general measures which have been introduced to overcome these disadvantages are addition of a large amount of a stabilizer or a lubricant, and addition of processability-improving agent of the alkyl acrylate type.
However, the former method leads to a higher cost of the material, a lower impact strength, and a lower heat distortion temperature, and thus is not desirable from the standpoint of the quality of the molded articles. The latter method of adding an alkyl acrylate as a processability-improving agent involves disadvantages such that the impact strength of the injection molded articles is lowered, and delamination of the resin may occur at a gate portion in separating the gate portion resin from the injection molded article at the completion of the molding, although this method is very useful because of a lower cost of the material and a smaller lowering of the heat distortion temperature. In particular, use of an increased amount of the processability-improving agent for enhancing the improvement effect will adversely decrease the impact strength and remarkably increase the delamination of the injection-molded articles.
The aforementioned disadvantages are considered to be caused by the use of a processability-improving agent of the alkyl acrylate type which is intended to improve the processability of extrusion-molding resins having approximately 200,000 to 100,000 of a molecular weight notwithstanding those of injection-molding resins having aproximately 100,000 to 50,000 of a molecular weight.
The inventors of the present invention noticed that (a) a remarkably high shear rate of 10,000/sec to 100,000/sec is applied to the resin during injection molding in comparison with that of 100/sec to 1,000/sec applied during extrusion molding and (b) the flame-retardant resin to which the processability-improving agent is to be added is not a resin having a usual molecular weight but a special flame-retardant injection-molding resin composed of a low-molecular-weight styrene resin and a low-polymerization-degree vinyl chloride resin. Based on this consideration, the inventors of the present invention made a comprehensive study to provide a fire-retardant injection-molding resin composition which contains an alkyl acrylate type of processability-improving agent without an accompanying decreased effect on thermal stability improvement, lowered impact strength, or delamination of injection-molded articles at the gate portion. Consequently, the inventors found surprisingly that incorporation of an alkyl acrylate copolymer of a specific composition into the resin composition can avoid a decrease in impact strength and delamination of the molded article at the gate portion, and yet can improve the thermal stabilizing effect as compared with a prior known resin composition, and completed the present invention.